All publications referenced herein are incorporated by reference to the same extent as if each publication or patent application was specifically and individually indicated to be incorporated by reference. The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Hair is a filamentous biomaterial that grows from follicles found in the dermis. Human hair has many textures, from fine to coarse, and from straight to curly. The outermost layer of the hair filament is called the cuticle and consists of translucent scales that cover the shaft. The cuticle protects the shaft from environmental damage. It is also sensitive to pH changes: at high pH the scales open, leaving the cortex exposed to environmental conditions. This is commonly exploited in hair processing, from straightening to coloring. Hair is composed primarily (around 88%) of keratin, a protein made up of chains of polypeptide helix coils. The polypeptides have a high cystine content that leads to extensive crosslinking via the disulfide linkage. The bonds are perpendicular to the helix coils, with one bond for every four turns of the helix. It is the crosslinking that gives hair its toughness and abrasion resistance, as well as defining its shape.
Curly hair is made of hair strands with irregular surfaces that mesh and tangle to make combing and management more challenging than with straight hair. It therefore has become popular among many individuals to relax or straighten hair in order to increase manageability and ease of styling.
The oldest form of hair straightening involves the use of strongly alkaline agents, at pH 12-14, to permanently convert the crosslinking disulfide bonds in cystine to lanthionine. During this harsh chemical treatment, the hair is mechanically maintained in a straight conformation. In a milder, alternative hair relaxation process, the cystine disulfide bonds are chemically reduced to produce temporary cysteine residues. Common reducing agents used in these processes include thioglycolic acid, ammonium thioglycolate, and bisulfite. These are typically used at high pH (e.g., 9.5). A neutralizer/fixative is applied in the second stage of the treatment, along with mechanical straightening and flat-ironing at 200° C. (400° F.), to restore the pH balance to the hair and to re-form the disulfide crosslinks in their new positions. This process produces permanent straightening, is damaging to the hair, and takes a relatively long time (4-6 hours) to perform.
The demand for temporary (1-6 months) hair straightening systems that remove the frizz from curly or wavy hair leading to a straighter form without the use of harsh chemicals has grown in recent years. The popular temporary hair straightening process known as escova progressiva was pioneered in Brazil. Known in the United States as “Brazilian hair straightening” or “keratin straightening,” this process can provide results lasting up to 5 months and uses high temperatures (typically 230° C., 450° F.), keratin lysate, and elevated levels (1-5%) of formaldehyde as its primary components. The highly toxic nature of formaldehyde, a proven human carcinogen, and the high temperatures used in the process raise legitimate concerns regarding its safety for stylists and clients. Other, similar treatments have emerged that replace the toxic formaldehyde with chemicals such as glyoxal. For example, Resnick et al. disclose a formulation and method relying on keratin lysate, a crosslinking agent such as a polyfunctional aldehyde, and styling with a flat iron to achieve curl minimization (1). This treatment, however, has the disadvantage of requiring high levels of heat (typically >200° C.) to be effective, and these high temperatures often cause an undesirable change in color of the processed hair (e.g., orange coloring). Syed et al. also discloses a curl minimizing composition based on keratin lysate, a crosslinking agent, and heat to achieve temporary hair straightening (2). However, that treatment, as described, is unlikely to achieve sustained hair straightening lasting in excess of several months with repeated hair washing. Considering the practical limitations and safety concerns relating to existing hair straightening treatments and methods, there is clearly a need in the art for a more effective and non-toxic means of hair straightening.